The present invention relates to an exhaust gas purification catalyst for purification of exhaust gases by burning particulates (solid carbon fine particles, liquid or solid high molecular weight hydrocarbon fine particles; hereinafter sometimes referred to as xe2x80x9cPMxe2x80x9d) contained in exhaust gases discharged from diesel engines, and exhaust gas purification materials using the catalyst.
Recently, it has become clear that particulates discharged from diesel engines are mostly less than 1 micron in their particle diameter and are apt to float in the atmosphere and readily taken into human bodies by breathing, and, besides, they contain carcinogenic substances such as benzpyrene. Thus, their effects on the human bodies have become serious. Under the circumstances, regulations against discharging of particulates from diesel engines are further tightened, and exhaust gas purification catalysts and exhaust gas purification materials which are capable of efficiently removing the particulates.
Hitherto, as one of the methods for the removal of particulates from exhaust gases, there is a method which comprises collecting particulates in exhaust gases using a heat resistant exhaust gas purification material having a three-dimensional structure, heating the exhaust gas purification material by a heating means such as a burner or an electric heater after rising of the back pressure, thereby burning the deposited particulates to convert them to carbon dioxide, and discharging the carbon dioxide to the outside.
However, the above method suffers from the problem that since the burning temperature of the particulates is high, a large energy is required for burning and removing the collected particulates and regenerating the filters. Further problem is that the filters undergo melting loss or cracking owing to the burning in a high temperature region and the reaction heat generated therein. Another problem is that the size of the purification apparatuses becomes large and the cost increases because special devices are required.
On the other hand, there are methods according to which the fine particles are subjected to burning reaction by the catalytic action of catalysts, thereby to perform burning and regeneration in the exhaust gas at the temperature of the exhaust gas without using heating means such as heaters.
As an exhaust gas purification material which supports a catalyst, there is a material which comprises a heat resistant three-dimensional structural body on which an exhaust gas purification catalyst is supported, and the particulates collected therein can be burnt at lower temperatures by the catalytic action of the exhaust gas purification catalyst.
If particulates can be burnt at the exhaust gas temperature using the above exhaust gas purification material having an exhaust gas purification catalyst supported thereon, there is no need to provide a heating means in an exhaust gas purification apparatus, and, thus, construction of the exhaust gas purification apparatuses can be simplified.
However, at present, for the exhaust gas purification materials on which an exhaust gas purification catalyst is supported, it is still difficult to burn sufficiently the particulates at the exhaust gas temperature, and a heating means must be used in combination. Accordingly, there has been desired development of exhaust gas purification catalysts on which is supported an exhaust gas purification materials having such a high catalytic activity as being capable of burning the particulates at lower temperatures.
As exhaust gas purification catalysts, those which contain oxides of metals such as copper and vanadium have been known to have relatively high activity.
For example, JP-A-58-143840 discloses xe2x80x9ca particulate purification catalyst comprising a combination of at least one member selected from copper and compounds thereof and at least one member selected from metals and compounds thereof capable of having a plurality of oxidation statesxe2x80x9d, and JP-A-58-174236 discloses xe2x80x9ca catalyst for purification of particulates contained in exhaust gases which comprises at least one member selected from vanadium and vanadium compoundsxe2x80x9d.
However, the exhaust gas purification catalysts disclosed in these patent publications have the problem that since the catalytic activity of the exhaust gas purification catalysts is not so high as capable of sufficiently burning the particulates at low temperatures of the exhaust gas temperatures, the particulates collected in the exhaust gas purification materials cannot be burnt at the exhaust gas temperatures, and, thus, a heating means must be used in combination.
Furthermore, JP-B-4-42063 discloses xe2x80x9can exhaust gas purification catalyst comprising a metal oxide such as of copper, manganese or molybdenum to which an alkali metal oxide and a noble metal are added, and a method for producing the samexe2x80x9d.
However, the exhaust gas purification catalyst disclosed in the above patent publication has the problem that the catalyst has an alkali metal oxide as a component, and the alkali metal oxide is inferior in heat resistance and is scattered by the heat of the exhaust gas or reacts with other catalyst components. Moreover, the exhaust gas purification catalyst has another problem that it is poisoned with sulfur oxide contained in the exhaust gas to cause deterioration of catalyst activity.
The present invention solves these problems encountered with the conventional techniques, and the object of the present invention is to provide an exhaust gas purification catalyst which has a high catalytic activity for burning of particulates, can exhibit sufficiently the respective catalytic characteristics, can sufficiently burn and remove particulates at a temperature close to the exhaust gas temperature, and is high in exhaust gas purification efficiency, and an exhaust gas purification material which can burn and remove particulates at a very high efficiency and is considerably excellent in endurance and economical efficiency.
The exhaust gas purification catalyst of the present invention for the solution of the above problems has a construction which contains a first catalyst component comprising an inorganic oxide having heat resistance and a transition metal oxide supported on the inorganic oxide and a second catalyst component comprising at least one alkali metal sulfate.
By allowing the catalysts having different functions to be separately present as mentioned above, their different catalytic characteristics can be sufficiently brought out and, furthermore, deterioration caused by reaction between the catalysts per se can be inhibited, and, as a result, deterioration of activity of the catalyst can be inhibited, whereby an exhaust gas purification catalyst of high activity can be obtained.
Moreover, by forming a catalyst component on the surface of a heat resistant inorganic oxide, the surface area of the catalyst increases, and thus the contact points with the particulates in the diesel exhaust gas increase. As a result, an exhaust gas purification catalyst of high activity can be obtained.
The exhaust gas purification catalyst of the present invention contains a first catalyst component comprising an inorganic oxide having heat resistance and a transition metal oxide supported on the inorganic oxide and a second catalyst component comprising at least one alkali metal sulfate. That is, the catalysts having different functions are separately provided, whereby their respective different catalytic characteristics can be sufficiently brought out, deterioration caused by reaction between the catalysts per se can be inhibited, thus deterioration of activity of the catalyst can be inhibited, and, as a result, an exhaust gas purification catalyst of high activity can be obtained.
Moreover, a catalyst component is formed on the surface of an inorganic oxide having heat resistance, whereby the surface area of the catalyst increases, thus the contact points with the particulates in the diesel exhaust gas increase, and, as a result, an exhaust gas purification catalyst of high activity can be obtained.
Furthermore, a catalyst component is formed on the surface of an inorganic oxide having heat resistance, whereby the necessary amount of the catalyst can be reduced, and an exhaust gas purification catalyst can be obtained at low cost, and this is highly economical.
In addition, a transition metal oxide catalyst supported on an inorganic oxide having heat resistance and a catalyst of an alkali metal sulfate are separately provided and the surface area of the catalyst is increased, whereby the contact points with the particulates in the diesel exhaust gas increases, and the catalyst activity can be increased.
Furthermore, a transition metal oxide catalyst supported on an inorganic oxide having heat resistance and a catalyst of an alkali metal sulfate are separately provided and the surface area of the catalyst is increased, whereby necessary and sufficient amounts of the transition metal oxide and the alkali metal sulfate can be reduced and an exhaust gas purification catalyst can be obtained at low cost, and this is highly economical.
Moreover, a transition metal oxide catalyst supported on an inorganic oxide having heat resistance and a catalyst of an alkali metal sulfate are separately provided, whereby the reaction between the catalysts per se due to the heat in the burning of particulates can be inhibited, the different catalytic characteristics can be sufficiently exhibited, and, besides, deterioration of catalytic activity can be inhibited to enhance the endurance of the catalysts.
The exhaust gas purification catalyst of the present invention may further contain a noble metal as a third catalyst component.
The noble metal may be supported on the surface of the inorganic oxide separately from the transition metal oxide. In addition, the noble metal may be supported on the surface of the inorganic oxide together with the transition metal oxide. The ratio of the first catalyst component and the second catalyst component has no special limitation, however, preferably the weight ratio of the first catalyst component/the second catalyst component is 0.01-50, more preferably 0.1-5. The ratio of the inorganic oxide and the transition metal oxide of the first catalyst component has no special limitation, however, preferably, the weight ratio of the transition metal oxide/the inorganic oxide is 0.001-2, more preferably 0.01-0.5. The ratio of the third catalyst component has no special limitation, however, preferably the weight ratio of the noble metal/the inorganic oxide is 0.0001-0.2, more preferably 0.001-0.1.
The construction of an exhaust gas purification catalyst and an exhaust gas purification material in one embodiment of the present invention will be explained below.
First, the inorganic oxide having heat resistance as a carrier will be explained. As the inorganic oxides having heat resistance which support the transition metal oxide and the noble metal, there may be used at least one inorganic oxide selected from Ta2O5, Nb2O5, WO3, SnO2, SiO2, TiO2, Al2O3 and ZrO2. Two or more of them may be used. By supporting the transition metal oxide and the noble metal catalyst components on the inorganic oxides, the surface area of the catalyst components is increased, the chance of contact with PM is increased, and the purification efficiency is improved. Furthermore, amount of the catalyst necessary for obtaining the same surface area can be reduced, and the cost can be decreased. The shape of the inorganic oxide has no special limitation, but spheres of 0.1-1000 xcexcm in particle diameter are preferred.
Next, the transition metal oxide catalyst will be explained. As the transition metals, mention may be made of Cu, Mn, Co, V, Mo, W, etc., and one or more oxides of one or more of these metals can be used. In this specification, transition metal oxide includes composite transition metal oxide.
Specific examples of these transition metal oxides are CuO, V2O5, CoO3, MnO2, MoO3, WO3, etc., and one or two or more of them can be used.
Oxides of Cu are especially preferred, and as the oxides of Cu, there may be used at least one compound selected from CuO, Cu2O, and Cu2O3.
The composite oxides are preferably composite oxides comprising Cu and V, and at least one compound selected from Cu5V2O10, CuV2O6 and Cu3V2O8 can be used. As other composite oxides, mention may be made of CuMoO4.
These transition metal oxides can efficiently burn and remove PM and enhance the catalyst activity. Furthermore, by using the composite oxides comprising Cu and V, it becomes possible to remove PM at a temperature near the exhaust gas temperature.
The method for supporting the transition metal oxide on the inorganic oxide has no special limitation, and, for example, the methods referred to in the examples given hereinafter can be employed.
Next, the noble metals will be explained. Examples of the noble metals are Pt, Pd, Rh, Ru, etc., and one or two or more of these metals can be used. These noble metals can reduce harmful components present in the exhaust gas together with PM, such as carbon monoxide, nitrogen oxides, hydrocarbons, etc. Furthermore, since the noble metals react with hydrocarbons or carbon monoxide in the exhaust gas at low temperatures, the exhaust gas temperature rises and the catalytic activity of the transition metal oxide catalyst for PM can be increased. Among these noble metals, Pt can highly efficiently burn, for example, SOF component, etc. other than the carbon components of PM, thereby to purify the exhaust gas, and thus Pt is especially preferred.
The method for supporting the noble metal catalyst on the surface of the inorganic oxide or on the surface of the transition metal oxide has no special limitation, and, for example, the methods mentioned in the examples given hereinafter can be employed.
The combination of the noble metal catalyst and the transition metal oxide catalyst, namely, the combination of platinum (Pt) as the noble metal and the composite oxide of copper (Cu) and vanadium (V) as the transition metal composite oxide which are supported on titania (TiO2) used as the inorganic oxides is very high in catalytic activity and is particularly preferred.
The transition metal oxide or noble metal supported on the surface of the inorganic oxide may be in the form of continuous layer or discontinuous stripes on the surface of the inorganic oxide. Further, the transition metal oxide or the noble metal may be in the dispersed state. That is, in the present invention, the transition metal oxide catalyst and the noble metal catalyst include the state of discontinuous stripes and the dispersed state.
As the alkali metals of the second catalyst containing an alkali metal sulfate, mention may be made of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), etc., and it is preferred to use one or two or more sulfates of these alkali metals.
Specific examples of the alkali metal sulfates are lithium sulfate, sodium sulfate, potassium sulfate, rubidium sulfate, cesium sulfate, etc., and cesium sulfate alone or a mixture of cesium sulfate and potassium sulfate is especially preferred. By using the alkali metal sulfates, deterioration of the catalyst components caused by the sulfur components in the exhaust gas can be prevented and the catalytic activity for PM can be exhibited at its maximum.
Moreover, the above exhaust gas purification catalyst can be supported on a three-dimensional structural body having heat resistance. The method for supporting the catalyst has no special limitation, and, for example, the methods mentioned in the examples given hereinafter can be employed. As given in the examples, the second catalyst component may be supported on the upper surface of the first catalyst component, additionally, the first catalyst component may be contained in the second catalyst component. The amount of the catalyst supported has also no special limitation, and can be optionally set depending on, for example, the size of the three-dimensional structural body. As the materials of the three-dimensional structural body, metals, ceramics, etc. are used.
As the metals, there may be used iron, copper, nickel, chromium, etc. each alone or as alloys of two or more of them in combination.
As the ceramics, there may be used cordierite, aluminum titanate, mullite, xcex1-alumina, zirconia, titania, silicon carbide, silica, silica.alumina, alumina.zirconia, etc.
The shape of the three-dimensional structural body having heat resistance on which the exhaust gas purification catalyst is supported has no special limitation, and there may be used ceramic honeycombs of wall-flow type, ceramic honeycombs, ceramic foams, metal honeycombs, metallic filters and metallic meshes of flow-through type, etc., and preferred are honeycomb-shaped filters of wall-flow type, and foams or metallic filters of flow-through type. Ceramic honeycombs of wall-flow type are especially preferred.
The materials of the honeycomb-shaped filters are not particularly limited, and metals, ceramics, etc. are used.
The shape of the foams has no special limitation, and examples are foam type filters having continuous pores in the three-dimensional direction.
The materials of the foams may be metals, ceramics, etc. and are not particularly limited, but ceramic foams of cordierite can be used suitably.
Foaming ratio of the foams is preferably 5-50/square inch, more preferably 10-30/square inch in terms of the number of pores.
The alkali metal sulfate formed and supported on the three-dimensional structural body having heat resistance may be in the state of a continuous layer or discontinuous stripes. Moreover, the alkali metal sulfate may be in the dispersed state.
That is, when the first catalyst component contained in the second catalyst component, more specifically, for example, powder particles of the inorganic oxide which support the transition metal oxide on their surface or powder particles of the inorganic oxide which support the noble metal on their surface are contained in the layer of the alkali metal sulfate on the three-dimensional structural body, these powder particles are not needed to be completely incorporated in the layer of the alkali metal sulfate, and they may be in the state of being dispersed in the area of the alkali metal sulfate which is in the state of discontinuous stripes or in the dispersed state.
In the present invention, the layer of the exhaust gas purification catalyst is not necessarily formed in such a state that the layer of the components is continuous, and at least the layer of the components can be present in the state of discontinuous stripes or in the dispersed state.
The present invention can be used for removal of particulates in the exhaust gases of not only automobile engines, but also engines of conveyance means such as cultivators, ships, trains, etc., industrial engines, combustion furnaces, boilers, etc.
The present invention is not limited to the above embodiments and variation can be made without departing from the spirit and scope of the invention.